In the field of semiconductor manufacturing there is a continuing demand for smaller device sizes (i.e., higher transistor counts per unit area) and increased processing power (i.e., faster processing speeds).
Because of their desirable resistance to oxidation, the use of precious metal alloys for stationary and moveable or sliding electrical contacts as well as test probes enjoys widespread use for applications such as slip ring brushes, semiconductor probes, potentiometric sensors, etc. For the past 60 years, Paliney™ 7 has been recognized as the benchmark alloy for applications requiring excellent oxidation resistance and formability in its fully age hardened condition. However its electrical conductivity is very low, being only about 5.5% IACS.
U.S. Pat. No. 5,833,774 to Klein et al. discloses compositions of silver/palladium/copper alloys which are used in such applications and describes a range of noble metal alloys, which when heat treated, can offer a range of hardness levels with electrical conductivity in a range of 12-16% IACS. Even though commercial alloys consistent with this teaching (Paliney™ H3C and Paliney™ C) have conductivity values are nearly three times that of Paliney 7 (see Table 2), they still fall short of the desired current-carrying capacity of many new applications. For example, for integrated circuit (IC) test probes with diameters below 100 microns, current levels that can be successfully used remain below 2 Amps because of excessive electrical heating (“New Generation of Probe Alloys”, Smith, et. al., IEEE SW Test Workshop, June, 2013). Another shortcoming of alloys within this family is their difficulty in being formed into complex, highly toleranced shapes when in the fully age-hardened condition.
U.S. Pat. No. 6,210,636 to Klein discloses a low cost silver/palladium/copper/nickel/zinc high strength alloy suitable for sliding electrical contact applications. However, because this alloy was developed to reduce its noble metal content and resultant cost by increasing its nickel and zinc content, its oxidation resistance is poor in comparison to alloys having higher noble metal contents. Additionally, for these alloys, the overall conductivity is generally below 10% IACS (Paliney™5, DNI website)
Although alloys in the Pd—Cu—Ag family have been studied since the 1950's (Raub and Worwag, Z. Metallkd., 1955, 46, 52-57), most of the published work has focused on documenting the possible phase relationships and establishing the effect of ordering on the electrical properties of the alloys. As shown in FIGS. 1A and 1B, ordering reactions are known to dramatically reduce the electrical resistivity. FIG. 1A contains the phase diagram for the Cu—Au binary alloys showing the presence of ordered stoichiometric phases, Cu3Au and CuAu. FIG. 1B (Barrett, 1952) illustrates the change in the electrical resistivity of the binary Au—Cu alloys as they are thermally treated to transform from the disordered to the ordered state. In the disordered state (background dotted line), the resistivity is minimized at each of the pure metal states and gradually increases as the two elements are mixed, reaching a maximum near the 50-50 at % level. However, by heat treating the alloys within the appropriate time-temperature regime, it is possible to create an ordered phase and minimize the resistivity at both the 25 at % and 50 at % Au levels. The resistivity values vary in a linearly symmetric fashion as the composition is varied in either direction from the stoichiometric values. This behavior is the generally accepted model for order-disorder transitions.
Palladium/copper alloy systems have also been subjects of technical papers and articles. A. Yu. Volkov, in “Improvements to the Microstructure and Physical Properties of Pd—Cu—Ag Alloys,” examined and reported on a range of compositions for the ternary alloy system. Volkov examined the effect of adding silver to a palladium-copper alloy with the primary focus on improving tensile strength. Although this work shows a positive impact on strength, as illustrated in Volkov, all the Ag additions also act to increase the resistivity, e.g., going to a resistivity of roughly 8.5 micro-ohm cm (20.3% IACS) for the Pd—Cu binary alloy going from a resistivity of roughly 11 micro-ohm cm (15.6% IACS) for an alloy with 12 at % Ag. This work does not present any significant understanding regarding how to simultaneously optimize both the mechanical and electrical properties.
Additionally, U.S. Pat. No. 7,354,488 introduces the use of Re in conjunction with other elements such as B, Ni and Ru to increase the strength of high Pd content wrought alloys. In the absence of the synergistic influence of these complementary elements, the data suggests it takes Re levels of at least 10% to reach hardness levels over 300 HK. These alloys typically have very low electrical conductivity values in the 5-8% IACS level. In these systems where the Pd levels are usually above 75 wt %, the Re is thought to be a solid solution strengthening agent and not participate on a second phase or ordering reaction. Re is also occasionally used as a grain refining additive in dental casting alloys, but at very low concentrations, typically below 0.5%.